1. Field of the Invention
The present invention relates to a photothermographic material. More specifically, the invention relates to a photothermographic material which exhibits improved surface physical properties.
2. Description of the Related Art
In recent years, in the field of films for medical imaging, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems” by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All of the patents, patent publications, and non-patent literature cited in the specification are hereby expressly incorporated by reference herein. In particular, photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region. (See, for example, U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924.) Further, the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system using photothermographic materials that has been made commercially available.
Methods of manufacturing such a photothermographic material utilizing an organic silver salt include a method of manufacturing by a solvent coating, and a method of coating an aqueous coating solution using an aqueous dispersion of fine polymer particles as a main binder followed by drying. Since the latter method does not require a process of solvent recovery or the like, a production facility therefor is simple and the method is advantageous for mass production.
In the case of the photothermographic material having an aqueous-based coated image forming layer utilizing organic silver salts described above, the use of hydrophobic polymer latex as the main binder for the image forming layer to avoid adverse influence by moisture on photographic properties is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-10670, and the additional improvement thereof leads to formation of a clear image. However, under various utilization conditions, such as storing the photothermographic material at high temperature and humidity, the surfaces of the photothermographic material are liable to be adhered to each other, whereby separation thereof often causes defects such as the image forming layer being scratched or peeled off, and therefore improvement is needed.
Problems such as the defects described above are easily generated especially when a hydrophilic polymer derived from animal protein (for example, gelatin) is used in the outermost layer. A method for improvement is disclosed in JP-A No. 2002-162712, but the improvement does not reach a sufficient level, and there are negative effects such as lowering of maximum density (Dmax) and deterioration in brittleness of the film. There is therefore a need in the art for improved photothermographic materials which do not exhibit the above negative effects.
JP-A No.2004-309641 discloses a photothermographic material in which a non-photosensitive layer including polymer latex having a fluorine atom is disposed as an outermost layer on the side of a support having thereon an image forming layer. However, the physical properties provided thereby do not reach sufficient levels required for the surface of the photothermographic material.